This invention relates to the field of forming coolant passageways in rocket thrust chambers or turbine airfoils.
A slotted niobium thrust chamber cylinder is provided, and the slots are filled with a filler material. An outer niobium overcoat is deposited over the outer surfaces of the cylinder and over the filler material and the filler material is thereafter removed by leaching to form the coolant passageways.
Future space propulsion systems will employ refractory metal thrust chambers. Chemical vapor deposition (CVD) is being used to enclose a slotted niobium liner. CVD niobium is deposited at a high temperature, at about 1200 degrees C., in a reactive gas environment. The critical step in the process is the selection and application of the later to be removed filler material in the niobium substrate liner slots. The numerous desired characteristics of the filler material are as follows: high-temperature melting point, moldable to complex flow channels with thermal and structural stability, metallurgically compatible with the niobium substrate liner at high temperature, chemically compatible with the CVD reactive gas environment, coefficient of thermal expansion similar to the substrate, will not degrade the bond of the deposited niobium, can be readily removed from the very small flow passages, leaving close tolerance, dimensionally controlled flow channels.
Earlier attempts to develop suitable temporary filler materials employed photoetched shaped and contoured strips of molybdenum for use with the niobium substrate material. Molybdenum met all the criteria, except it could not be successfully acid leached, to form the hollow coolant channels, after the CVD of the niobium outer cover. Also, thermal stresses caused the strips to shift. Further attempts were made to develop better filler materials. Metal fillers and high-temperature salts were evaluated. Powdered metal (-325 mesh) were selected to permit molding directly into the variable width and depth channel slots. This was accomplished by mixing the powders with an acrylic cement, filling the slots, and sintering the assembly to vaporize the acrylic cement and densify the metal powder. Powdered molybdenum was tried because of its solid solubility with niobium but, acid leaching of even the low-density powder was unsuccessful. Readily dissolved copper was tried in malleable wire and powder forms, but its low melting point limited the CVD process temperature and inhibited bonding of the niobium outer cover. Only iron powder fulfilled most of the requirements. However, formation of niobium-iron intermetallic compounds restricted thermal exposure time, and it also appeared that iron contamination of the substrate lands degraded the niobium deposit bond.